【英语时差8,16】旧书为什么变黄（下）

（单词翻译）

The earliest paper, made of white cotton rags beaten by hand consisted almost entirely¹ of long cellulose fibers². As the demand for paper increased, paper makers³ used chlorine to bleach⁴ colored rags. Later, heavy machinery⁵ made it possible to turn wood into paper pulp⁶. But any chlorine bleach left in the paper eventually turns into hydrochloric acid. And so, too, does the lignin, which holds the cellulose fibers together in wood. Other acid-producing compounds were used to treat the paper, and even air pollution from coal-burning factories increased the acid-content of paper. All that acid breaks the chemical bonds between the cellulose molecules⁷, making the paper brittle⁸ and turning it yellow. Today, many important documents are printed on acid-free paper, but in the meantime, library preservationists are working on ways to save millions of books before the acid in the paper turns them to dust.